1. Field of the Invention
Embodiments of the present invention generally relate to methods and apparatus to sealably close and open a tubular within an oil and gas wellbore. More particularly, embodiments of the present invention generally relate to methods and apparatus for creating a fluid seal used to produce a pressure differential that is utilized to actuate a hydraulic tool downhole.
2. Description of the Related Art
Hydrocarbon wells typically begin by drilling a borehole from the earth's surface to a selected depth in order to intersect a hydrocarbon-bearing formation. Steel casing lines the borehole formed in the earth during the drilling process. This creates an annular area between the casing and the borehole that is filled with cement to further support and form the wellbore. Thereafter, the borehole is drilled to a greater depth using a smaller diameter drill than the diameter of the surface casing. A liner may be suspended adjacent the lower end of the previously suspended and cemented casing. This liner overlaps the casing enough to provide gripping engagement between the casing and liner when hung or suspended and extends to the bottom of the borehole.
In the completion of oil and gas wells, downhole tools are mounted on the end of a drill support member, commonly known as a work string. The work string may be rotated or moved in an axial direction from a surface platform or rig. Illustrative work strings include drill strings, landing strings, completion strings and production strings. Wellbore tubular members such as casing, liner, tubing, and work string define the fluid flow path within the wellbore. Commonly, a need arises to temporarily obstruct one or more of these fluid flow paths within the wellbore. An obstruction that seals the fluid flow path allows the internal pressure within a section of the tubular conduit to be increased. Hydraulically driven tools operate from this increased internal pressure. For example, a hydraulically operated liner hanger can be utilized to hang the liner to the well casing. However, a subsequent step in the completion of the oil or gas well may require the obstructed fluid path to be reopened without requiring the removal of the tubing string from the well in order to clear the obstruction.
Sealably landing a ball on a ball seat provides a common means of temporarily blocking the flow through a tubular conduit in order to operate a hydraulic tool thereabove. Thereafter, increasing pressure above the ball seat causes a shearable member holding the ball seat to shear, releasing the ball seat to move down hole with the ball. However, this leaves the ball and ball seat in the well bore, potentially causing problems for subsequent operations.
Another method of reopening the tubular conduit occurs by increasing the pressure above the ball seat to a point where the pressure forces the ball to deformably open the seat and allow the ball to pass through. In theses designs, the outer diameter of the ball represents the maximum size of the opening that can be created through the ball seat. This potentially limits the size of subsequent equipment that can pass freely through the ball seat and further downhole without the risk of damage or obstruction.
Hydraulic tools located above a ball seat are set to operate at a pressure below the pressure that opens or releases the ball seat. Internal pressures can become quite high when breaking circulation or circulating a liner through a tight section. In order to avoid premature operation of the tool at these times, the pressure required to open or release a ball seat needs to be high enough to allow for a sufficiently high activation pressure for the tool.
For example, predetermined open or release pressures that are set when the ball seat is assembled can exceed 3000 psi. Stored energy above the ball seat results from the compressibility of the fluid and any entrained gases along with the energy stored from the ballooning in the tubular conduit. Therefore, releasing or opening a ball seat by increased pressure can cause the ball to pass through the drill pipe at a relatively high velocity and prematurely release ball seats or shift sleeves located downhole. The large surge pressure created by the ball seat's release can also undesirably damage formations or cause hydraulic tools below the ball seat to actuate prematurely.
Even with precision manufacturing and extensive quality control, occasional malfunctions occur in the activation mechanisms of the tool and the release or opening mechanisms of the ball seat due to these devices' dependency on hydraulic pressure. For example, when the ball seat opens or releases at a lower pressure than planned, the hydraulically operated tool may not have activated or completed its function. Similarly, if the hydraulically operated tool does not function at its desired pressure, the ball seat may reach its release or opening pressure before the tool is activated.
Since the ball seat is a restriction in the wellbore, it must be opened up, moved out of the way, or located low enough in the well to not interfere with subsequent operations. Commonly, the ball seat is moved out of the way by having it drop down hole. Unfortunately, this may require the removal of both the ball and ball seat at a later time. Ball seats made of soft metals such as aluminum provide easier drill out; however, they may not properly seat the ball due to erosion caused by high volumes of drilling mud being pumped through the reduced diameter of the ball seat. Interference from the first ball seat being released downhole may also prevent the ball from sealably landing on another ball seat below. Current collet style mechanisms open up in a radial direction when shifted past a larger diameter grove. However, these ball seats are more prone to leaking than the solid ball seats, and the open collet fingers exposed inside the tubular create the potential for damaging equipment used in subsequent wellbore operations.
Wiper plugs often possess ball catchers that capture the ball when it is released. Thus, they must withstand the shock force imparted when the ball is released and subsequently caught. If a ball seat is alternatively placed in or at the bottom of the wiper plugs, then they must withstand the added force of the pressure acting on the ball seat. However, wiper plugs are built from materials that can be easily drilled in order to minimize drill out times. This requires a balance of strength versus drillability. Placing the ball seat above the wiper plugs provides an acceptable solution only if the released ball and ball seat do not interfere or obstruct the tubular passage during subsequent wellbore operations.
Therefore, there exists a need for an improved apparatus and method for temporarily blocking a fluid path in a wellbore in order to operate a hydraulic tool. There is a further need for a ball seat that does not depend on hydraulic pressure for release, that releases without causing a surge in the tubular below, that can be placed above the wiper plugs, that withstands an impact of a ball released above, that withstands erosion, and that leaves a substantially unobstructed passage through the bore once opened.